Paints or coatings used in conventional transfer roll coatings for coating paper, in which pigments and adhesives are the main components typically are highly viscous compared to other coating colors, and characterized in that coating weights are generally low. Pigments which may be used in a typical formulation include inorganic pigments such as kaolin, clay, precipitated calcium carbonate, ground calcium carbonate, titanium oxide, aluminum hydroxide, satine white, barium sulfate, magnesium oxide, talc and colloidal silica; as well as organic pigments such as plastic pigments and white urea resin pigments, which are generally used in admixture of two kinds or more. Adhesives which may be used include water-soluble polymers such as casein, oxidized starch, phosphoric esterized starch, soybean protein and carboxymethyl cellulose, as well as synthetic rubber latexes such as styrene-butadiene rubber (SBR), methyl butadiene rubber (MBR) or methyl styrene butadiene rubber (MSBR), and acrylic latexes. The adhesives (also referred to as binders) typically are used in an amount of about 10-50 weight % of the water-soluble macromolecule, and 10-30 weight % of the latex, based on 100 weight % of the pigment. Furthermore, additives such as anti-foaming agents, dispersants, water-proofing agents, preservatives, coloring agents or lubricants may be added.
The solids content of the transfer roll coating color is typically about 40-55 weight %, and the ratio between the pigment and the adhesive (pigment:binder ratio) is such that the adhesive is about 30-50 weight % to the pigment, which means that the amount of the adhesive is high. Furthermore, starch adhesives and latex adhesives often are used together and the ratio of the starch adhesive can be as high as 20-50 weight % to the pigment. The starch adhesives generally used have a viscosity of at least 3,000 centipose (cps), generally 10,000 cps or higher, as determined at a solids content of about 30 weight %, a temperature of about 50 .degree. C. and 60 rpm by means of a No. 4 rotor of a Brookfield.TM. viscometer. In addition, the amount of coating on one side of the paper generally is less than 5 g/m.sup.2, more often less than 3 g/m.sup.2, for an effective coating.
Transfer roll coating is known to be suitable for coating sizing liquids onto paper because a light weight coating is obtained. Transfer roll coating of paper has many advantages, such as (1) the device is compact, (2) the coating can be carried out with wide webs, (3) on-machine coating can be carried out, (4) less paper break occurs, (5) a continuous operation is possible for several days, (6) coating on both sides of the paper is easily achieved, and (7) the difference between the front and back surfaces of the product are minimal. However, there are also some disadvantages, such as (1) the feel of the coated surface often is inferior to that obtained by blade coating, (2) the characteristics of the coating may be limited, and (3) maintenance of the transfer rolls is necessary.
The coated surface obtained by transfer roll coating often has a specific stripy coating pattern, and transfer roll coating may not give an even, smooth and glossy coated surface, compared to that which can be obtained with blade coating. The appearance of the stripy pattern can be improved by increasing the coating weight, but a split surface pattern such as orange peel, or an aventurine appearance, may then appear which reduces the quality of printability which can be obtained when the transfer roll coated paper is printed. For these reasons, in spite of its many advantages described above, transfer roll coating has been used only for applying sizing liquid coatings or light weight coatings where a high coated amount is not necessary, or for preparation of a low-grade coated paper where printability is not critical.
There has recently been a market trend toward higher quality coated papers. For example, in art papers, so-called super art papers with more gloss and superior printability have appeared; and papers having higher opacity, stiffness and gloss are required even in the customary grades such as superlight weight coated papers. This tendency toward improved quality is similar in low-grade coated papers. To obtain the requisite improvement in smoothness and gloss of the coated paper, a super calender or a machine calender has hitherto been used, and means such as an increase in the calender pressure and temperature are used to obtain coated papers with higher gloss. However, if these means are applied to transfer roll coated paper, gloss may be improved somewhat, but calipers become short, and qualities such as opacity, whiteness, stiffness and ink acceptability are lowered.
An object of the present invention is to provide a transfer roll coating color with which an even coated surface without any undesirable surface patterns, as well as high-gloss and smooth coated surface can be obtained using low pressure calendering conditions, without affecting any of the advantages of transfer roll coating. These advantages include stable runnability at high speed on machine, and a coated paper product having excellent printability, comparable to coated papers obtained by the blade coating method.
Generally, if the coating weight is increased, a smoother surface can be obtained. The coating weight in transfer roll coating is controlled by adjusting the nip pressure between the rolls, the circumferential speeds of each roll, and the solids content in the coating color. Therefore, the present inventors considered that an increase in the coating weight could solve the above-mentioned problems, and studied the nip pressure between rolls, the circumferential speeds and the solids contents in the coating color. In the case where the nip pressure was lowered to enlarge the gap between rolls, the coating weight was increased, but an undesirable ring-form pattern was produced in the coating. In the case where circumferential speeds were differentiated, the feeling of the coated surface worsened. It was thus found that the coating weight may be only slightly controlled in practice by differentiating the nip pressure or circumferential speeds. Therefore, control of the coating weight is mainly a function of the solids content in the color. To obtain coatings having high coating weights, the solids content of the coating color should be increased. However, if the solids content is increased using the materials used in conventional transfer roll coating colors, the viscosity of the color is also increased, thereby producing a split pattern on the applicator roll where the coating is applied. The said pattern not only appears on the coated surface to damage the feel of the coated surface, but also produces during the coating process boiling of the coating, gum-up and scattering of color mists, by which some of the advantages of transfer roll coating, namely, wide webs, high coating speed and stable operation on the machine, are lost.
By further study, the present inventors found that if the solids content is about 58 weight % or higher, and the viscosity of the color at the said solids content is about 1,500 cps or less determined at 60 rpm by using a No. 4 rotor of a Brookfield.TM. viscometer, transfer roll coating can be carried out to give a highly glossy and smooth coated surface under low pressure calendering conditions without affecting the advantages of the transfer roll coating process, namely, high speed and stable runnability on the machine. Furthermore, use of such coating colors provides coated papers with excellent printing quality, comparable to that obtained by the blade coating method. Furthermore, we found that a coating color having a solids content of about 58 weight % or higher, and a viscosity determined at 60 rpm by using the No. 4 rotor of a Brookfield.TM. viscometer of 1,500 cps or less, can be obtained by use of a low viscosity adhesive, and that a coated paper having both good feel and appearance of the coated surface can be obtained using this coating color to complete the present invention.